A system wherein flexible ducts from different directions are brought over a low support before rising as seen in GB-A No. 20ll506. Other designs and configurations for flexible risers have been described by Coflexip in their brochure "Dynamic Flexible Risers" printed in September 1985. These include five configurations named as follows:
(a) free-hanging PA1 (b) lazy S PA1 (c) steep S PA1 (d) lazy wave PA1 (e) steep wave.
The last four all have mid-water supports. In addition "Offshore Engineer" of February 1985 shows a form of lazy S configuration wherein the lower leg, below the mid water support, runs back under that support i.e. towards the region under the vessel.
It is likely that many semi-submersible floating production systems will in future have at least some of their production wells positioned directly beneath the vessel for two reasons:
produced fluids can flow more readily without suffering from the slugflow, hydrate, wax and emulsion problems associated with subsea flowlines;
workover and other subsea equipment maintenance can be performed directly without requiring a second vessel.
The steep S and steep wave configurations, which are designed to be used when wells are beneath the vessel, are difficult to install and require extra connection points at the riser base. It is desired therefore to reduce the number of connections to a minimum and to make installation as simple as possible. It may be necessary to perform the installation without using divers.